The present invention relates generally to object detection and position determination, and more particularly to object detection and position determination by reflected global navigation satellite signals.
Global navigation satellite systems (GNSSs) can determine locations with high accuracy. Currently deployed GNSSs include the United States Global Positioning System (GPS) and the Russian GLONASS. Other GNSSs, such as the European GALILEO system, are under development. GNSSs are used in a wide range of applications, such as surveying, geology, and mapping.
In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the navigation receiver; the line-of-sight signals are referred to as direct signals. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The navigation receiver measures the time delays of the received signals relative to a local reference clock. Code measurements enable the navigation receiver to determine the pseudo-ranges between the navigation receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the navigation receiver and the satellites due to various error sources and due to variations in the time scales of the satellites and the navigation receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the code coordinates and time scales at the navigation receiver.
Navigation receivers can also receive satellite signals that are reflected from the general environment, such as the ground, and from specific structures, such as buildings. These reflected signals, referred to as multipath signals, degrade the accuracy of the code coordinates determined by the navigation receiver. For high-accuracy measurements, navigation receivers are designed to reduce the effect of multipath signals. For examples, specially-designed antennas can reduce the multipath signals initially received, and signal processing algorithms can separate multipath signals from direct signals.
Reflected signals, however, can be used to detect and determine the position of specific objects. One method is described in U.S. Patent Application Publication No. 2008/0129587 (“DiEsposti”). A GPS-enabled receiver receives direct signals from GPS satellites and reflected signals from a reflector. The direct signals are isolated from the reflected signals based on their time delays and amplitudes. The position of the receiver is determined from the direct signals, and the position of the reflector is determined from the reflected signals. The described method, however, assumes a substantially isotropic reflector, and, furthermore, does not account for reflections from the general environment.